By 2266, Riley was serving as navigator aboard the Enterprise as a lieutenant. In that year, Riley was one of the first crewmembers to fall victim to an unusual disease which caused those affected to act out and lose their inhibitions. Under the disease's influence, Riley commandeered the ship's engineering section, and rerouted all command functions there, prohibiting the rest of the crew from controlling the ship. This posed a significant danger to the Enterprise, as it was locked in an unstable orbit of the planetPsi 2000, and in the process of spiraling down towards the surface. Proclaiming himself "Captain Kevin Thomas Riley," he deactivated the ship's engines and regaled the crew with repeated renditions of the song "I'll Take You Home Again, Kathleen." Eventually, Captain Kirk and crew were able to regain control of the ship, and institute a full-power restart of the engines, narrowly avoiding destruction of the Enterprise. (TOS: "The Naked Time")

Kirk talks Riley out of killing Kodos

Later that year, Riley was serving in the communications department of the Enterprise. When Captain Kirk came to believe that an actor which had been brought aboard the ship was, in actuality, Kodos the Executioner, who had murdered Riley's family twenty years previously, he had Riley transferred back down to the engineering section in an attempt to keep Riley out of harm's way. Riley, who was unaware of Kirk's true motivations, viewed the transfer as a demotion, as he had previously been transferred up from engineering. Despite Kirk's attempts to protect him, Riley was almost killed by Lenore Karidian, Kodos' daughter, who was attempting to eliminate all survivors of the massacre who could identify Kodos. After surviving the attempt on his life, Riley learned of Kodos' presence on the ship, and attempted to take revenge by killing him with a hand phaser, although he was eventually talked out of the attempt by Kirk. (TOS: "The Conscience of the King")

Despite wearing lieutenant rank insignia in "The Naked Time", his rank wasn't established in the final draft script of that episode, which instead referred to him merely as a navigator. The same script also commented, "He is no singer... should hold the pitch but not much more...."

Bruce Hyde was cast as Riley because he had a contract with Desilu. "My family has never traced our genealogy back, and I don't even know that I'm actually Irish, so that had nothing to do with my getting the part of Kevin Riley!" he exclaimed. "But I was lucky: [....] I got to sing and joke around, which was unusual for the series." Because he was mainly a theater actor who hadn't done much television when he first appeared as Riley, however, the "joking around" part didn't come easily to Hyde. "I remember doing all of that stuff [as Riley] in engineering, staggering back and forth, and singing 'Kathleen'," he continued. "What I was doing was supposed to be funny, but everybody was just quiet [because they were supposed to be]. That was difficult for me, so the director, Marc Daniels, started choking me kind of playfully, trying to get me to loosen up and make me comfortable on the set." (Star Trek: The Original Series 365, pp. 054 & 055)

In the script of "The Conscience of the King", the character whose parents were murdered by Kodos was named "Lieutenant Robert Daiken" (colloquially abbreviated as "Daik"). However, when Hyde was cast in the role, the character was renamed to fit with his previous appearance as Riley in "The Naked Time". This explains why, whereas Riley is a navigator in that episode, he is a communications officer in "The Conscience of the King".

Despite appearing in only two installments of TOS, Riley became one of the most popular Enterprise crewmembers in the series. (The Star Trek Compendium, 4th ed., p. 37)

During the twentieth anniversary of the original series, it was revealed that, "Admitting he enjoys the Star Trek movies, Hyde comments that if the producers ever decide to write the character back into the film series, he thinks he wouldn't mind returning, though he reserves the right to decide until he's asked." (Starlog, November 1986, p. 60)

For the thirtieth anniversary of the original series, the name and assignment "Kevin Riley, Navigator" was printed on a sign above a corridor door as an in-joke for production on DS9: "Trials and Tribble-ations". (Cinefantastique, Vol. 29, Nos. 6/7, p. 77)

Kevin's son Terrance Riley (β) was featured in the novelCrossover, where he released CaptainScott after Scott stole the USS Yorktown in search of Spock, who had been captured with a group of Unification believers. He released Scott on the promise that he would retrieve Spock and bring back the Yorktown to the museum it was in. He did so because he felt that his father owes Spock for his life after the events of "The Naked Time". It was also noted that Kevin must have had children relatively late in his life, and that his son commanded a Galaxy-class starship.

Riley appeared in the novel The Galactic Whirlpool, as a prominent member of the team sent to contact inhabitants of a lost generation ship. Hyde commented on the novel and the role of his character therein, stating "I thought it was great. I've enjoyed David Gerrold's work before and was honored to be selected by him as a subject," Hyde says, adding, "And by the way–Kevin Riley isn't back. He has never been gone. He has been studying, learning, risking, growing, getting ready to bust out into the universe!" (Starlog, November 1986, p. 60)

He also appeared in the novel Probe, set after Star Trek IV: The Voyage Home, where he was a protégé of Ambassador Sarek and later became an ambassador himself during negotiations with the Romulans. According to the book Probe, his father was executed when he was very young. His career as an ambassador was also explored in Foul Deeds Will Rise, when he was assigned to supervise crucial disarmament talks with the aid of the Enterprise-A; during the novel, he encountered Lenore Karidian once again, suspecting that she had suffered a relapse when two of the delegates were killed in a manner implicating her, but accepted that she had changed when Kirk identified the true culprit and Lenore risked her life to destroy a protomatter warhead.